1. Field of the Invention
The invention relates to a surface acoustic wave converter for converting an electric signal into a surface acoustic wave. More particularly, the invention relates to a surface acoustic wave converter in which a frequency band limitation is reduced. The invention also relates to a surface acoustic wave converter having a unidirectional performance in which a frequency band limitation can be reduced and an excited surface acoustic wave can be propagated in one direction. The invention also relates to a surface acoustic wave device using those surface acoustic wave converters and to a communication system using such a device.
2. Related Background Art
Generally, a surface acoustic wave (SAW) device applies an electric field to an interdigital transducer (IDT) formed on a substrate having a piezoelectric performance made of LiNbO.sub.3, LiTaO.sub.3, quartz, or the like and executes a signal generation using a piezoelectric effect such that the quartz is deformed or a signal detection using a piezoelectric effect opposite to the above effect. FIG. 7 shows a constructional example of the IDT. In the diagram, reference numeral 1 denotes an input terminal; 2 a ground electrode; and 3 a piezoelectric substrate. As a piezoelectric substrate 3, either a piezoelectric substrate made of a piezoelectric substance or a piezoelectric substrate in which a piezoelectric substance is formed on a non-piezoelectric substrate can be used. A surface acoustic wave is excited by a signal which is inputted from the input terminal 1.
According to the IDT shown in FIG. 7, since the surface acoustic wave progresses in both directions (right direction and left direction in the diagram) of the IDT, in case of a construction using only the surface acoustic wave in one direction, a loss occurs. Therefore, in order to reduce an insertion loss as much as possible in the surface acoustic wave device using the IDT as a converter on the inputting side or a converter on the outputting side, there is a case where a unidirectional converter, a converter having therein a reflector, a reflecting array, or the like is used in the converter on the inputting side or the converter on the outputting side. An example of the unidirectional converter is shown in "The Television", Vol. 30, Section 6, pages 458-459, 1976.
FIG. 3 is a diagram showing an example of a unidirectional converter using a 90.degree. phase-shifter. In the diagram, each of reference numerals 30, 31, 32, and 33 denote an electrode group which includes three electrode fingers to which a signal is inputted and in which a width of each electrode finger and a width of space between the adjacent ground electrodes are equal to .lambda./4 (.lambda. is a wavelength of a surface acoustic wave at a center operating frequency). Reference numerals 34 and 36 denote ground electrodes each having an electrode width of .lambda./2); 35 a ground electrode having an electrode width of .lambda.; 37 and 38 bus bars connected to the electrode fingers; and 39 a 90.degree. phase-shifter.
In FIG. 3, the signal which is inputted to the electrode groups 31 and 33 from the input terminal 1 through the 90.degree. phase-shifter 39 via the bus bar 38 has a phase difference of 90.degree. with a signal which is inputted to the electrode groups 30 and 32 at a center frequency of the surface acoustic wave that is excited by the IDT. Therefore, as for the surface acoustic wave which progresses to the right in FIG. 3, the phase conditions among the electrode groups are matched. On the other hand, as for the surface acoustic wave which progresses to the left, the phases between the adjacent electrode groups don't coincide. Thus, a unidirectional converter can be realized.